It's not that high-efficiency, well-insulated dwellings aren't vital to both saving money and being kind to the Earth. But if you're trying to stay warm, layering up with long underwear—specifically the newer, synthetic kind—is mathematically more efficient than messing with the heat or walls.

Low-Tech Magazine offers a fantastically geeky read on the thermal efficiency of personal insulation versus household heat. You'll learn about "clo units," the measure of insulation needed to keep a resting person comfortable at 70 degrees Fahrenheit. You'll also see that wearing a form-fitting layer of long undies can keep you much warmer, due to the heat generated by thin air layers, movement, and other thermodynamics best explained by the author:

According to the US Air Force Survival Book, one layer of long underwear (long pants + long-sleeved t-shirt) equals a clo-value of 0.6, while two layers of long underwear add a clo-value of 1.5. In other words, the clo-value more than doubles by using only two layers. Combine this outfit with a typical business suit (or a similar, more comfortable clothing ensemble), and thermal insulation rises to 2.5 clo, enough to keep a couch potato indefinitely comfortable at a temperature of only 12.7° Celcius (55°F) - far below the common indoor temperatures of today. This clothing ensemble could yield a reduction in energy use for space heating of up to 80 percent.

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Old-school, natural ingredient long underwear is also pretty handy, the article points out, but the newer, UnderArmor-style synthetics seem to do a better job at trapping heat. If you've got an under-clothing recommendation, or can't believe the numbers you're seeing, let's hear about it in the comments.